1. Field of the Invention
The present invention relates to a hollow fiber membrane module for filtering a liquid. More particularly, the present invention relates to a hollow fiber membrane module which is suitable for use in a solid-liquid separating process for obtaining a pure liquid from a suspension.
2. Prior Art
Hollow fiber (also called "hollow fiber membrane") has an excellent filtering ability and by virtue of its structure presents a large surface area as required for filtration. Therefore, it has heretofore been widely employed for various uses, for example, production of industrial water of high purity, filtration of a gas, etc. In addition, a method has heretofore been practiced wherein a hollow fiber membrane module is immersed in water to be treated, which has a suspended matter concentration in the range of from several thousands [mg/L] to ten thousands [mg/L], to directly obtain filtered water therefrom.
The hollow fiber membrane module has generally been produced as follows. To facilitate production, a multiplicity of hollow fibers are bundled at at least one end thereof and are secured in a securing part, and a water collecting part is provided adjacent to the securing part. Further, it has been conventional practice to form a hollow fiber membrane module by using a bundle of hollow fibers which are arranged in a U shape because the number of hollow fibers needed is halved if bundled in such a configuration. In this type of hollow fiber membrane module, the hollow fibers are in close contact with each other because they are tied in the securing part provided at one end thereof.
If a liquid to be treated which has a high concentration of suspended matter, e.g., a suspension in a biological treatment tank, is filtered by using the above-described hollow fiber membrane module, the suspended matter enters the spaces between the multiplicity of hollow fibers and remains there, so that no suspension can pass thereby. Accordingly, it is likely that a group of hollow fibers will become stuck to each other as a result of suspended matter attached thereto. In such a case, it is difficult for raw water to enter and contact the inside fibers of the bundle, so that the available filtration area decreases, resulting in a lowering in the filtering function. Further, in the hollow fiber membrane module that employs a U-shaped hollow fiber bundle, the U-shaped end portion of each hollow fiber, which is provided opposite to the free end portion of the hollow fiber that is secured in the securing part, can usually move freely. Therefore, each hollow fiber moves freely in the current of a liquid, so that the hollow fibers are likely to become tangled with each other and form a cluster.
To overcome these disadvantages, Japanese Utility Model Post-Exam Publication No. 2-13067 proposes a unit wherein a hollow fiber support member having a ring-shaped configuration, for example, is disposed to engage the U-shaped end portions of hollow fibers arranged in a bundle so that the hollow fibers are prevented from moving freely.
The proposed unit suffers, however, from the following problems. Since the unit is designed so that the hollow fiber membrane module is disposed in a straight pipe of the unit, the size of the hollow fiber support member, on which the hollow fibers are supported, is restricted by the size of the straight pipe and hence, the support member is configured so that it can be accommodated in the straight pipe. That is, the diameter of the support member is almost the same as that of the securing part of the membrane module. For this reason, the hollow fiber support member only functions to disperse the hollow fibers in a parallel arrangement. However, even with this unit, clustering of the hollow fibers in the hollow fiber bundle is not substantially obviated, and it is impossible to enlarge a spacing between adjacent hollow fibers. Accordingly, the suspended matter entering the space between the hollow fibers is likely to stay there, causing the hollow fibers to adhere to each other. Thus, the above-described problems remain unsolved.
The clustering of the hollow fibers in the hollow fiber bundle may be prevented, for example, by increasing the diameter of the ring-shaped hollow fiber support member. By such means, it is possible to provide a slight increase in the spacing between the U-shaped end portions of the hollow fibers of the U-shaped hollow fiber bundle. However, longitudinally extending portions of each U-shaped hollow fiber engaged with the ring-shaped hollow fiber support member are still positioned close to each other. In addition, there is no improvement in preventing the clustering of the hollow fibers near the securing part, to which the free end portion of the bundle is attached.